Water de-sanding (i.e., removal of grit) is an important process in water and wastewater treatment plants. In most contemporary sewage (wastewater) treatment plants, the first treatment process is screening and removing relatively large material, and the second process is removing grit. “Grit” includes sand and materials of similar size and density such as pieces of grout, cement, and tile. If not removed, grit can damage subsequent treatment and process equipment and needlessly occupy volume in digesters.
Various means have been used to remove inorganic and putrescible material in the grit collection operation while maintaining organic and putrescible material suspending in the flow of water passing out of the grit collection, so that such material might subsequently be removed and treated and disposed of without causing health problems or environmental impact. These means employed include velocity control via channel design, weirs and baffles, mechanical mixing, and aeration.
Essentially all grit collectors rely on gravity or an induced force such as centrifugal force, which can be expressed in “G's,” equivalent to some multiple of the Earth's gravitational force. Some such devices, such those taught by Weis, U.S. Pat. No. 4,767,532, and Schloss, U.S. Pat. No. 5,569,379 (both incorporated by reference), show mechanically agitated grit collectors where the agitators provide a relatively constant velocity to the tank contents. Grit from these and similar collectors is removed by a pump and disposed of through a concentrator, cyclone, decanting device, or the equivalent. Usually the grit is pumped to a grit concentrator (or hydrocyclone) wherein the grit is separated from typically around 95% of the water by modern involute fed, 20 degree cyclone, then the thickened grit slurry is discharged into a grit washer/classifier whence it is conveyed from the water and washed by the rotation of a conveying screw. While such systems are effective, they are relatively energy inefficient. It will thus be appreciated that a superior means to remove grit from a grit collector is a significant improvement in the state of the art.